1. Field of the Invention
The present invention relates to a direct type fuel cell power generator using methanol or a methanol aqueous solution, etc. as a fuel, and more particularly, to a direct type fuel cell power generator in which a stable output can be obtained by improving a shape of a flow path of a flow path plate through which a fuel flows.
2. Description of the Related Art
A fuel cell is provided as a device which converts chemical energy having a fuel such as hydrogen, hydrocarbon, or alcohol by electrochemical reaction. This device is expected as a power generator of high efficiency and low pollution type.
In such fuel cells, a solid polymer type fuel cell for which an ion exchange resin film is used as an electrolyte is provided as a fuel cell whose development has been accelerated as a power source for electric automobile or a power supply for housing in recent years. The solid polymer type fuel cell supplies a fuel gas containing hydrogen to an anode electrode side and an oxygen gas or air to a cathode electrode side. In the anode electrode and cathode electrode, reactions shown in (Formula I) and (Formula II) respectively occur, and an electromotive force is generated.Anode electrode: 2H2→4H++4e−  (Formula I)Cathode electrode: O2+4H++4e−→2H2O  (Formula II)
That is, by means of a catalyst inside of the anode electrode, an electron and a proton are generated from hydrogen, and the electron is captured by an external circuit. After the proton is conducted inside of a proton conductive electrolyte film, when the conducted proton reaches the cathode electrode, the proton reacts on the electron and oxygen on the catalyst inside of the cathode electrode, and then, water is generated. Power is generated by such an electrochemical reaction.
On the other hand, in recent years, attention has been paid to a direct type methanol fuel cell. FIG. 59 shows a structure of an electromotive portion unit in the direct type methanol fuel cell. The direct type methanol fuel cell is constituted by sandwiching a proton conductive electrolyte film 7 (for example, a perfluorocarbon sulfonic acid based ion exchange film is used, and Nafion manufactured by Du Pont Co., Ltd. or the like is preferably used) between an anode electrode 3 and a cathode electrode 6. Each of the electrodes is composed of substrates 1 and 5 and catalyst layers 2 and 4. The catalyst layers each are constituted by a catalyst or a carbon black to which a catalyst has been carried being dispersed to a perfluorocarbon sulfonic acid resin. In general, as a catalyst, there is generally used a precious metal catalyst or an alloy of the metal. In many cases, the catalyst is used by carrying it on a carrier such as a carbon black. As a catalyst for the anode electrode, a Pt—Ru alloy is preferably used. Alternatively, as a catalyst for the cathode electrode, Pt is preferably used. In order to drive this fuel cell, methanol and water are supplied to the anode electrode side, and an oxygen gas or air is supplied to the cathode electrode side, whereby the reactions shown in (Formula III) and (Formula IV) occur with the anode electrode and cathode electrode, respectively.Anode electrode: CH3OH+H2O→CO2+6H++6e−  (Formula III)Cathode electrode: (3/2) O2+6H++6e−→3H2O  (Formula IV)
That is, by means of a catalyst in the anode electrode catalyst layer, an electron, a proton, and carbon dioxide are generated from methanol and water, and the generated carbon dioxide is discharged into air. The electron is externally captured as a current. In addition, the proton moves on the proton conductive electrolyte film, reaches the cathode electrode, and reacts on an electron and oxygen, and then, water is generated. Power is generated based on this electrochemical reaction.
In this direct type methanol fuel cell power generator, a closed circuit voltage is generally 0.6 V to 0.8 V, and in actual power generation with a charge current, the voltage drops to a voltage close to 0.5 V. Therefore, in order to obtain a voltage at which operation of an electronic circuit or electric equipment is compensated for, it is required to electrically connect a plurality of electromotive portion units. Hence, there is need to stack the plurality of electromotive portion units, and provide a flow path shape or pipe for uniformly supplying a fuel to these units, and a variety of proposals have been made.
Many flow paths or pipes can be roughly divided into two structures, i.e., a parallel type flow path structure and a serial type flow path structure. In the parallel type flow path structure, the pipes or flow paths guided from a fuel container housing a fuel therein are branched by the number of electromotive portion units. In the serial type flow path structure, one flow path circulates a plurality of electromotive portion units sequentially.
However, in the former case, a dispersion of a fuel supply state with respect to each electromotive portion unit, the dispersion being caused by branching of the flow paths or pipes, is prone to occur, and there is a need to make further contrivance for decreasing such a dispersion. In the latter case as well, since a fuel is consumed serially in a plurality of electromotive portion units, there occurs a difference in output due to a fuel concentration difference in electromotive portion units positioned at the first half of the flow paths and in electromotive portion units positioned at the latter half of the flow paths. Also in this case, there is a need to design a fine flow path shape for decreasing the difference.
In addition, as a method for stacking a plurality of electromotive portion units and flow path plates, there is widely used a bipolar structure in the anode electrodes or cathode electrodes in the electromotive portion units are arranged in a unidirectional manner. In this bipolar structure, a flow path plate separating electromotive portion units is formed of a member made of an electrically good conductor, a fuel flow path is applied to one face of one flow path plate to supply a fuel, an oxidizing agent flow path is applied to the other face to supply an oxidizing agent, and these flow path plates are merely stacked alternatively in the electromotive portion units, whereby an electrical series state can be easily obtained. That is, electrical wiring for making serial electrical outputs from a plurality of electromotive portion units can be eliminated, thus making it possible to simplify a stack structure.
However, in actuality, in many cases, there is provided means in which a plurality of stack units of the number of stacks for which a mechanical strength or spatial restriction is compensated are disposed in parallel, each of which is electrically connected. For example, there is proposed a structure such that conductive flow path plates are insulated and aggregated with each other by using an insulating member. In ensure downsizing of this bipolar type stack, reduction of a flow path plate itself in thickness is the most effective except an element which depends on an electromotive portion unit itself, and a study has been made from structural and material points of view.
In order to structurally reduce the thickness of a flow path plate, there are considered a method for reducing the depths of anode and cathode flow paths and a method for reducing the thickness of a layer for partitioning the anode and cathode flow paths. In the former method, the depths of the flow paths are restricted by a pressure loss in flow path. Theoretically, it is possible to remarkably reduce the depths of the flow paths as long as a burden on a pump is ignored. However, in actuality, consideration must be taken into power generation efficiency and machining precision in an overall system including power consumed in a pump. The latter method is restricted by a fuel for a material or permeability for an oxidizing agent, and the strength of a material is restricted as film thickness is reduced.
Moreover, an attempt has been made to reduce a flow path plate in thickness from a material point of view. In general, as a material for flow path plate, there is often used a carbon which is a material having electrical conductivity. However, it is impossible to provide a pure carbon of 1 mm to 2 mm or less in thickness from the viewpoints of strength, permeability, and machining precision. Therefore, a slight amount of resin is permeated or mixed, whereby a material with the improved characteristics is used. However, if a rate of a nonconductive component other than carbon is increased, in general, it is difficult to provide such a characteristic compatible with a strength of resin or plastics suitable to small molding.
Because of this, there is proposed that a metal is used as a flow path plate in view of solving a program with strength or permeability in the above-described carbon based flow path plate. However, the flow path plate is provided as a portion which comes into contact with a fuel or oxidizing agent and an electrode portion and captures a current. Thus, a metal used as a material for flow path plate must have sufficient corrosion resistance. Metals available from a chemical point of view include precision metals such as gold, platinum, rhodium, iridium, ruthenium and the like. A flow path plate using these metal materials are hardly considered to be industrially applied from a cost efficiency. Therefore, in general, in forming a metal flow path plate, there is employed a technique for applying coating using the above precision metal onto an entire surface of a base material made of titanium or partial alloy which is a base metal having slight corrosion resistance. However, also in the thus prepared flow path plate, if a scratch of pinhole size occurs during electrode tightening, corrosion is considered to advance from such a scratched portion. In view of the above-described cost efficiency, currently, it is considered advantageous to use a carbon as a material for flow path plate rather than a metal.
As has been described above, a variety of attempts have been made to reduce a bipolar type stack in thickness from structural and material points of view, but remarkable improvement has not been achieved. In such a circumstance, as one of the methods for structurally reducing the stack in thickness, in recent years, there has been proposed a mono-polar type stack structure in which only an oxidizing agent or fuel is supplied to one flow path plate, and only a cathode electrode or an anode electrode is arranged on both faces of the flow path plate.
In the mono-polar structure, as compared with the bipolar structure, there is a disadvantage that an electrical series state according to a plurality of electromotive portion units cannot be easily formed merely by stacking them because the orientations at both ends of the electromotive portion units are not uniformed in a stacking direction. On the other hand, since only either of the oxidizing agent and fuel is supplied to one flow path plate, there is no need to make top and bottom flow paths independent of each other. Therefore, the mono-polar structure is structurally advantageous in that the thickness of partitioning the top and bottom flow paths can be eliminated. In addition, in a flow path equivalent to a depth equal to that in the bipolar structure as well, a wet edge length is reduced, and thus, a pressure loss of the flow path is expected to lower, making it possible to further reduce the depth of the flow path.
Accordingly, a mono-polar type stack structure is expected as a stack structure of a fuel battery power generator oriented to a portable information terminal, the stack structure requiring downsizing in particular. Further, in the case where such an application is considered, there is expectedly proposed a mono-polar type stack structure for direct type methanol fuel cell, in which there is a high possibility that a direct type methanol fuel cell which does not require a complimentary device such as a vaporizing device or refining device is used.
In the direct type methanol fuel cell, a methanol aqueous solution is consumed at an anode electrode, and hydrocarbon that is a reaction product at the anode electrode is generated as air bubbles. In addition, the volume of carbon dioxide that is the generated gas is several times as compared with a methanol aqueous solution that is a liquid to be supplied. The volume expansion of carbon dioxide in a flow path is one of the main causes which prevents the flow of the methanol aqueous solution in the flow path. Once prevention of the flow of the methanol aqueous solution inside of the flow path, such prevention causes a fuel supply rate-determining at the anode electrode, and a high charge current density cannot be obtained.
That is, this denotes a lowered output of a direct type methanol fuel cell, and such an output cannot be recovered until carbon dioxide retained in the flow path has been swept. This program with a gas-liquid double layer flow may occur in the flow path at the cathode electrode side. However, the problem is much more serious than a problem which occurs inside of the flow path at the cathode electrode side by virtue of reasons such as small liquid volume change rate than gas and greater inter-wall frictional force. That is, the above problem is more serious in a direct type methanol fuel cell for supplying a liquid fuel than a solid polymer type fuel cell (PEM, PEFC) in which gaseous hydrogen is supplied as a fuel to the anode electrode, and further, no gaseous product is obtained. A flow path design from this point of view is important in proposing a mono-polar type flow path plate for direct methanol fuel cell.
First, in general, a cross section of a flow path is reduced in order to achieve a smooth flow of a methanol aqueous solution inside of the flow path in the direct methanol fuel cell. This is because carbon dioxide generated inside the flow path is easily pushed out by efficiently increasing a flow rate of the fuel flowing the flow path. Further, a serpentine type flow path exhibiting a shape in which a narrow flow path is folded back many times is well used as a flow path of a direct type methanol fuel cell in order to deliver a fuel to an entire face of an electromotive portion unit in a state in which the cross section of the flow path is reduced.
In particular, since this serpentine type flow path can be easily formed as a bipolar type flow path plate, in forming the bipolar type flow path plate, the serpentine type flow path is often used. Furthermore, in order to increase power generation efficiency, a comb type protrusion partitioning the adjacent flow paths in an opposite manner is reduced in width so as to increase an area in which the electromotive portion unit and methanol aqueous solution come into contact with each other.
However, if the comb type protrusion is extremely reduced in width in order to increase power generation efficiency, the outer-most power collecting portion of an electrode in the electromotive portion unit is porous. Thus, air bubbles of carbon dioxide expanding therefrom short-circuits at the adjacent flow paths, and a pressure is not applied correctly in a direction in which the flow path advances. Because of this, there occurs a problem that a fuel is retained at a flow path portion which is short-circuited and through which no air bubbles pass. Conversely, there occurs a problem that, if a fuel short-circuit occurs, carbon dioxide is retained. Therefore, in general, the width of the comb shaped protrusion structure is often designed primarily by about 1 mm.
In other words, in order to ensure proper fuel supply to the bipolar type flow path plate in the direct type methanol fuel cell, it is desirable that a serpentine type flow path of about 1 mm in width of the comb type protrusion structure be used. Further, it is required to push an electrode face of the electromotive portion unit against a flow path plate by applying a proper pressure.
However, a similar flow path structure cannot be used for a mono-polar type flow path plate. This is because, in the serpentine type flow path produced so as to penetrate both faces of the flow path plate, the comb type protrusion structure is established in a floated state only at one small portion from the periphery of the flow path plate, and even under a pressure in the flow path which is not so problematic in the bipolar type flow path plate, the short-circuit of carbon dioxide or fuel easily occurs. Also, a method which solves this problem is not proposed yet. As a flow path shape of a mono-polar type flow path plate which has been studied in recent years, there is merely used a simple structure in which a plurality of linear flow paths are arranged in parallel. Accordingly, there is expectedly proposed a flow path shape for improving power generation efficiency and a flow path plate structure or material for achieving such a shape.
The above-described problems with the mono-polar structure are similar in the case of a flow path plate made of metal which can be easily formed. As the cutting faces of the flow path plate are very large in number, making it further difficult to increase the uniformity of corrosion resistance processing. In addition, as in the bipolar structure, in the case where the electromotive portion units are arranged in parallel in a planer direction of the flow path plate, a complicated structure via an insulating member must be unavoidably provided.
The following problem with the above-described direct type methanol fuel cell has occurred. That is, the direct type methanol fuel cell is expected to be used as a power source of a portable electronic device from a height of energy density of methanol which is a liquid fuel. In addition, there is no need for fuel pressurization from the viewpoint of a liquid fuel. Further, there is a low possibility that a fuel leakage occurs from a gap between a flow path and an electromotive portion unit as compared with a solid polymer type fuel cell which uses hydrogen as a fuel. Therefore, unlike a fuel supply flow path of the solid polymer type fuel cell, it is considered possible to provide a comparatively complicated flow path structure or flow path disposition. However, there is not proposed a flow path structure in a direct type methanol fuel cell power generator which solves problems with the parallel type flow path and the serial type flow path, respectively.
Moreover, as long as a flow path plate consisting essentially of a carbon for power collection is used, rapid development and production of a small sized fuel cell power generator for portable device becomes obstacle due to necessity of improvement and development of a carbon material for reducing one flow path plate in thickness; necessity of technique for integrated molding using an insulating material for making the plates in parallel in a planar direction; or complication or the like in which plural types of members are required in a production process.